1. Field of the Invention
The present invention relates to test apparatus, and, in particular, to apparatus suitable for use in conjunction with a passive optical network (PON) system.
2. Related Art
The applicant's International patent applications numbers GB 90/01758 (WO91/08623) and GB 91/00100 describe a TPON (Telephony on a Passive Optical Network) system designed to carry data via a Bit Transport System (BTS). The TPON system includes a head-end station having a time division multiple access (TDMA) master which assembles incoming exchange traffic into TDMA frames. These frames are broadcast via an optical stage onto a fibre network. Each of a number of terminations connected to the network recognizes and responds to an appropriately addressed portion of the data in the broadcast frame and ignores the remainder of the frame.
The header portion of each frame includes regions used for the monitoring and control of the system. In particular, as described in the above mentioned applications, upstream TDMA frames may include a ranging check pulse transmitted by a termination. In response to this ranging check pulse the head-end station transmits control signals to the termination to modify the timing of transmissions from the termination so as to correct for any variations in the time of receipt of the transmissions. The transmission from a termination is arranged to be delayed, with respect to the receipt of a TDMA frame, by a certain amount as determined by the head-end station such that the terminal transmission is in its correct place in the upstream TDMA frame received at the head-end station. The head-end station is typically arranged to generate an alarm signal if the terminal transmission delay as measured by the receipt of the ranging pulse falls outside a predetermined range. The head-end station may also transmit a diagnostic signal onto the network, as described in our International patent application number GB 91/01815.
While monitoring and control procedures of the type outlined above are effective once the TPON system is fully installed and running they are not in themselves capable of providing all the diagnostic functions necessary for a termination whilst that termination is being installed. Moreover those diagnostic functions which can be carried out provide an output in the form of alarm signals or other indications of the state of the network only at the head-end station. The relevant information is not then available directly for use by, for example, an engineer installing a termination at a position remote from the head-end station.